The demand for high-speed data communications has grown tremendously over the past several years. Mobile communications and internet access are two of the market drivers that have stimulated demand for data communications solutions around the world. Users have become attracted to mobile communications. Consequently, cellular/PCS technologies have experienced rapid growth. Similarly, the wired side of the network has experienced rapid growth and users are continuing to ask for additional speed. This need for speed is driven by several items, including the desire for remote office locations to appear to operate as quickly as one's main office environment and the need for higher speed internet access. Technologies are being developed and deployed to make this a reality. However, as with all new technologies, certain deficiencies in overall system operation need to be addressed and overcome if the markets are to realize their full potential.
One of the major problems with communications systems today is power consumption. Although much improved by yesterday's standards, excessive power consumption reduces the utility of mobile communications solutions by minimizing the battery life or the time between recharging sessions (in the case of rechargeable batteries). Most wired communications systems typically have access to some form of AC power. However, even wired communications systems must be concerned with the excessive dissipation of power for a variety of reasons. First, there are excessive costs to the communications provider ensuing from the consumption of power provided by the electric company. This cost concern is so serious that certain regulated industries have converted the long-term cost of electricity into a capital expense. Second, maintaining circuits at a reasonable operating temperature adds a host of design constraints, including the inclusion of additional fans, air conditioning, heat sinks and space for thermal ventilation. These constraints significantly increase the material, labor and maintenance costs associated with the system. Third, excessive heat may restrict the density of equipment, thereby increasing the size of the facility hosting the system and/or limiting the number of customers that can be served by a fixed size facility. This may require a service provider to construct new buildings in order to accommodate demand for their service. Accordingly, reducing the power consumption in communications systems can be a key aspect of any system design.
These concerns have been addressed to a large degree through advances in the digital domain through semiconductor integration and processing. Advances in integration have provided improved performance and reduced power consumption over the years by integrating functionality into a single chip. For example, integration of this type in many instances has eliminated the need to drive external buses and the associated capacitative nature of these buses. Advances in process technology continue to reduce power consumption as switching transistor geometries and capacitances are reduced as process technology "shrinks" the internal dimensions of semiconductor devices. Electric fields become higher as devices shrink, requiring the use of smaller supply voltages for semiconductor devices. These improvements will continue over time as advances carry technology to more circuits in smaller devices.
In the analog domain, however, little progress has been made to reduce the overall power consumption for certain classes of functions that are necessary in the communications world. Traditional amplifier design continues to lag behind the digital curve in terms of techniques to reduce overall power consumption. From cell phones to high-speed internet access technologies like ADSL, linear amplifiers are often required to deliver communications services to the end user.
Over the years, a variety of techniques have been used to improve the power efficiency of amplifiers. These techniques are well known and referred to as class A, B, AB, or C operation. These terms are used to describe the amount of time that an amplifier spends "in conduction." Class A amplifiers operate over the entire cycle of an input signal; class B amplifiers operate over slightly less than 180 degrees, and class C amplifiers are used in applications that conduct over a small portion of the input cycle. Class C amplifiers are non-linear and are typically used in applications in which input signals have constant amplitude envelopes. Class C applications are typically operated in a "tuned" environment that utilizes external tuned circuits to remove any unwanted harmonics associated with the highly nonlinear amplification caused by the class C conduction angles.
For applications that require minimal distortion, class A, and class AB (a combination of class A and class B operation that provides for efficiency and low distortion) amplifiers are typically the technologies of choice for most designers.
Class D, G and H emerged from efforts to improve power efficiency. Class D amplifiers use switching transistors and pulse width modulation (width is proportional to amplitude of input signal) techniques along with passive integrators to "reconstruct" a filtered representation of the input signal by integrating the series of "pulses." As the transistors in this "amplifier" were either on or off, the power dissipation in the amplifier was minimal. Unfortunately, the audio performance was poor and distortion was quite high. Class G amplifiers were an improvement on class D technology from an audio quality point of view; although their efficiency was not as good as the class D devices. Class G amplifiers attempt to reduce the amount of power dissipated in the amplifier's transistors. Typically, amplifiers must be able to amplify a wide range of input signals. This requires the amplifier to have large power supply "rails" (high-voltage supplies that connect across the output transistors that are used to "drive" the load). When the amplifier is amplifying small signals, there is a small AC voltage that is developed across the load; there is also a current that is passing through the load. This same current is passing through the output transistors. Unfortunately, the power transistors drop the balance of the supply voltage across themselves, while supplying load current. A simple calculation reveals that these devices dissipate a substantial amount of power. Indeed, a compromise for having large voltage rails is that efficiencies become quite poor as signal levels are reduced. Ideally, the power supply rails could be switched between two supply voltages depending on the level of the input signal. Small signals do not invoke the switching of the output devices to the higher voltage (or supply) rail. Instead, small input signals tend to be amplified using the lower voltage of the two output rails. As the input signal increases in amplitude, a point is reached where the lower railed output amplifier can no longer keep up with the increasing amplitude of the input signal. When this occurs, the output transistors automatically switch to the higher voltage rail. This allows the amplifier to respond to various signal levels without consuming excessive amounts of power.
Class H amplifiers tend to operate similarly to class G amplifiers with a primary exception being that class H amplifiers tend to have continuously variable rails that track the input voltages. When circuits monitoring the input detect the need for additional power (by virtue of increased input amplitude), special circuits rapidly increase the power supply voltage across the output transistors. Class H can provide even higher potential amplifier efficiencies (at the expense of complexity), than class G amplifiers. These amplifiers have been used for typical audio applications whose signals range from DC to 20 kHz.
Engineering present-day communication systems, where efficient use of power is a primary consideration, involves the design of system pieces using different types of integrated circuits and amplifier types. Typically, the integrated circuits and amplifier types are selected based upon sets of specifications defined for each of the system pieces. Consequently, each system piece carries with it the advantages and disadvantages of the associated power-consuming integrated circuit and amplifier type. Until the disadvantages are completely eliminated, there will continue to be a need for circuit arrangements and methods of using power more efficiently for communication systems and its system pieces.